1. Field of the Invention
The present invention relates to an ink jet recording head which ejects ink to a recording medium to record an image.
2. Related Background Art
An example of a conventional ink jet recording head (hereafter, this may be abbreviated a “recordinghead”) is shown in FIGS. 7A, 7B, 8A and 8B. FIGS. 7A, 7B, 8A and 8B are enlarged sectional views near a discharge port 105 where ink is discharged. Below the discharge port 105, a pressure chamber 103 in which a heater 102 is provided, a nozzle portion 101 which makes the pressure chamber 103 and discharge port 105 communicate, and an ink flow path 106 for supplying ink to the pressure chamber 103 are provided. Ink supplied to the pressure chamber 103 through the ink flow path 106 is heated by heat generated by the heater 102, and is discharged by the pressure of a bubble, which is generated in the ink at that time, from the discharge port 105 through the nozzle portion 101.
The nozzle portion 101 of the recording head shown in FIGS. 7A and 7B has a constant area of a section which is orthogonal to an ink ejection direction. On the other hand, in the nozzle portion 101 of the recording head shown in FIGS. 8A and 8B, an area of this section becomes large as it is close to the pressure chamber 103. Hereafter, the nozzle portion 101 shown in FIGS. 7A and 7B may be called a “straight nozzle” and the nozzle portion 101 shown in FIGS. 8A and 8B may be called a “tapered nozzle” for distinguishment. Here, the ink flow resistance of a straight nozzle is large, and hence, its energy efficiency of ink ejection is low. Therefore, in order to raise the energy efficiency of ink ejection, a tapered nozzle with small flow resistance becomes mainstream.
For example, when a distance OH from the discharge port 105 to a top face of the heater 102 is 75 μm and the height H of the ink flow path is 20 μm, the thickness (length) of the nozzle portion 1 of both of the straight nozzle and tapered nozzle become 55 μm. In this case, the inertance and viscous resistance of each nozzle portion 101 become as shown in Table 1.
TABLE 1StraightTapered nozzlenozzleTaper 5°Taper 12°Taper 19°NozzleInertance1.12E−018.04E−025.72E−024.37E−02portionInertance100725139ratio (%)Viscous2.28E−041.22E−046.82E−054.51E−05resistanceViscous100543020resistanceratio (%)PressureCeiling335829072010743chamberportionarea (μm2)Ceiling100876022portionarearatio (%)
The inertance and viscous resistance of the nozzle portion 101 act as resistance at the time of discharging ink, and when these are large, an ejection energy efficiency falls. The inertance and viscous resistance are expressed by the following formulas, respectively.Inertance M (kPa/(μm3/μs2))
  M  =      ρ    ⁢                  ∫        0        OP            ⁢                          ⁢                        ⅆ          x                /                  s          ⁡                      (            x            )                              where,
OP: thickness of nozzle portion
S(x): ink flow path sectional area in position of distance x from lower edge of nozzle portion (μm2)
ρ: specific gravity of ink
Viscous resistance R (kPa/(μm3/μs))
  R  =      η    ⁢                  ∫        0        OP            ⁢                        D          ⁡                      (            x            )                          ⁢                                  ⁢                              ⅆ            x                    /                                    S              ⁡                              (                x                )                                      2                              where,
D(x) is a shape factor of a nozzle, and when a nozzle is a rectangular solid:D(x)=12.0×(0.33+1.02×(a(x)/b(x)+b(x)/a(x)))when a nozzle is a cylinder:D(x)=8π
OP: thickness of nozzle portion
S(x): ink flow path sectional area in position of distance x from lower edge of nozzle portion (μm2)
η: ink viscosity (Pa·s)
In addition, since the inertance and viscous resistance in Table 1 are used for relative comparison, they are obtained by simple calculation.
Specifically, inertance is calculated on condition of specific gravity ρ=1, and, viscous resistance is calculated on conditions of coefficient of sectional form of nozzle=1 and viscosity η=1e−3 Pa·s. This is common to all the values of inertances and viscous resistances described below. In order to obtain strict inertance, it is necessary to use the specific gravity of ink to be used, and in order to obtain the strict viscous resistance, it is necessary to calculate using a coefficient of sectional form D(x) adapted to the viscosity η of ink and a cross-sectional form of a nozzle to be used.
As shown in Table 1, it is understood on a straight nozzle that its inertance and viscous resistance are large and it is inefficient. On the other hand, on a tapered nozzle, both of inertance and viscous resistance become small as a taper angle is enlarged. Specifically, at 5° of taper angle, inertance becomes 72% and, viscous resistance becomes 54% to a straight nozzle. In addition, at 12° of taper angle, the inertance becomes 51%, which is nearly a half, and the viscous resistance becomes 30% to the straight nozzle. Furthermore, at 19° of taper angle, the inertance becomes 39%, and the viscous resistance becomes 20%, which is ⅕, to the straight nozzle. Thus, it is possible to raise an ejection energy efficiency sharply in a tapered nozzle by enlarging a taper angle.
Nevertheless, in a tapered nozzle as shown in FIGS. 8A and 8B, a ceiling portion area of the pressure chamber 103 shown by hatching in the figure becomes small as a taper angle becomes large (as to specific numerical values, refer to Table 1). The ceiling portion area of the pressure chamber 103 decreases to 87% of a straight nozzle at 5° of taper angle, decreases to 60% at 12° of taper angle, and decreases sharply to 22% at 19° of taper angle. Since the ceiling portion area of the pressure chamber 103 acts as resistance to the approximately horizontal motion of ink to the ceiling portion when a bubble disappears, the motion loss of the bubble in a bubble disappearing process becomes large, and an impulse force at the time of the bubble disappearing becomes weak as this resistance becomes large. In the tapered nozzle with small flow resistance, since the kinetic energy of ink in a horizontal direction in the pressure chamber 103 becomes large in addition to the kinetic energy of the ink in the nozzle portion 101 being large at the time of the bubble disappearing, the impulse force generated at the time of bubble disappearing also becomes very large. As a result, the impulse force generated at the time of the bubble disappearing, i.e., the impulse force generated at the time of cavitation collapse, becomes large, and there has been a resulting problem of the heater 102 being easily damaged.